An improved composite surface model for the radar backscattering cross section of the ocean surface 1. Theory of the model and optimization/validation by scatterometer data

An improved composite surface model for the calculation of the normalized radar backscattering cross section (NRCS) of the ocean surface at moderate incidence angles is presented. The model is based on Bragg scattering theory. A Taylor expansion of the NRCS in the two-dimensional surface slope yields nonzero second-order terms which represent a first approximation for the effect of the geometric and hydrodynamic modulation of the Bragg scattering facets by all waves that are long compared to these facets. The corresponding expectation value of the NRCS varies with the wave height spectral density of all these waves, and it depends in a well-defined way on frequency, polarization, incidence angle, and azimuthal look direction of the radar. We show that measured NRCS values at frequencies ranging from 1 GHz (L band) through 34 GHz (Ka band) and wind speeds between 2 and 20 m/s can be well reproduced by the proposed model after some reasonable tuning of the input ocean wave spectrum. Also, polarization effects and upwind/downwind differences of the NRCS appear to be relatively well represented. The model can thus be considered as an advanced wind scatterometer model which is based on physical principles rather than on empirical relationships. The most promising field of application, however, will be the calculation of NRCS variations associated with local distortions of the wave spectrum by surface current gradients or wind effects.

[1]  M. Longuet-Higgins,et al.  Radiation stresses in water waves; a physical discussion, with applications , 1964 .

[2]  G. Whitham.  A general approach to linear and non-linear dispersive waves using a Lagrangian , 1965, Journal of Fluid Mechanics.

[3]  J. Wright.  A new model for sea clutter , 1968 .

[4]  J. Wright,et al.  Microwave scattering and the straining of wind‐generated waves , 1975 .

[5]  W. Grantham,et al.  The SeaSat-A satellite scatterometer , 1977 .

[6]  G. Valenzuela.  Theories for the interaction of electromagnetic and oceanic waves — A review , 1978 .

[7]  K. Hasselmann,et al.  The two-frequency microwave technique for measuring ocean-wave spectra from an airplane or satellite , 1978 .

[8]  F. Riedel,et al.  Proposed Model for the Elevation Spectrum of a Wind-Roughened Sea Surface , 1979 .

[9]  H. Masuko,et al.  Airborne microwave rain-scatterometer /radiometer , 1982 .

[10]  W. Plant.  A relationship between wind stress and wave slope , 1982 .

[11]  W. Pierson,et al.  The relationship between wind vector and normalized radar cross section used to derive SEASAT‐A satellite scatterometer winds , 1982 .

[12]  Archival of aircraft scatterometer data from AAFE RADSCAT missions , 1983 .

[13]  W. Alpers,et al.  A theory of the imaging mechanism of underwater bottom topography by real and synthetic aperture radar , 1984 .

[14]  J. Mitchell,et al.  Flight measurement and analysis of AAFE RADSCAT wind speed signature of the ocean , 1984 .

[15]  M. Donelan,et al.  Directional spectra of wind-generated ocean waves , 1985, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[16]  H. Masuko,et al.  Measurement of microwave backscattering signatures of the ocean surface using X band and Ka band airborne scatterometers , 1986 .

[17]  W. Plant.  A two-scale model of short wind-generated waves and scatterometry , 1986 .

[18]  G. J. St-Cyr,et al.  A radar ocean imaging model for small to moderate incidence angles , 1986 .

[19]  M. Donelan,et al.  Radar scattering and equilibrium ranges in wind‐generated waves with application to scatterometry , 1987 .

[20]  G. J. St-Cyr,et al.  Comparison of a new radar ocean imaging model with SARSEX internal wave image data , 1987 .

[21]  D. Lyzenga,et al.  Full‐spectrum modeling of synthetic aperture radar internal wave signatures , 1988 .

[22]  P. Saffman,et al.  Modulation of radar backscatter from the ocean by a variable surface current , 1988 .

[23]  Ocean Windfield Measurements With A Rotating Antenna Airborne C-band Scatterometer , 1989, 12th Canadian Symposium on Remote Sensing Geoscience and Remote Sensing Symposium,.

[24]  M. Banner.  Equilibrium Spectra of Wind Waves , 1990 .

[25]  R. Beal.  Directional Ocean Wave Spectra , 1991 .

[26]  Paul Snoeij,et al.  The polarization-dependent relation between radar backscatter from the ocean surface and surface wind vector at frequencies between 1 and 18 GHz , 1991, IEEE Trans. Geosci. Remote. Sens..

[27]  V. Wismann.  Ocean Wind Vector Measurements By The Rotating Antenna C-band Scatterometer During The Ers-1 , 1992, [Proceedings] IGARSS '92 International Geoscience and Remote Sensing Symposium.

[28]  D. Kasilingam,et al.  The validity of the composite surface model and its applications to the modulation of radar backscatter , 1992 .

[29]  A. Fung,et al.  A backscattering model for ocean surface , 1992, IEEE Trans. Geosci. Remote. Sens..

[30]  Bernd Jaehne,et al.  Two-dimensional wave number spectra of short wind waves: results from wind-wave facilities and extrapolation to the ocean , 1992, Optics & Photonics.

[31]  VIERS-1 Final report phase 3 , 1993 .

[32]  David L. T. Anderson,et al.  Wind retrieval and ERS-1 scatterometer radar backscatter measurements , 1993 .

[33]  W. Alpers,et al.  A three‐scale composite surface model for the ocean wave–radar modulation transfer function , 1994 .

[34]  J. Apel.  An improved model of the ocean surface wave vector spectrum and its effects on radar backscatter , 1994 .

[35]  W. Alpers,et al.  An improved composite surface model for the radar backscattering cross section of the ocean surface: 2. Model response to surface roughness variations and the radar imaging of underwater bottom topography , 1997 .